What happened
On 9 August 1996, a Boeing 747-236B, registration G-BDXH, was taxiing toward the runway at London Gatwick for a scheduled passenger flight. During the standard pre-flight control checks, the crew noticed the lower section of the rudder became stuck at a 14-degree right deflection. Almost immediately following this jam, the aircraft experienced a loss of fluid from the No 2 hydraulic system. The flight was aborted, and the aircraft returned to the terminal gate for inspection. The investigation confirmed that the lower rudder Power Control Unit (PCU) and its input linkage had sustained significant damage, leading to the aircraft being removed from service.
The investigation
Investigators examined the removed PCU and found a circumferential crack in the casing near the ram end, which had extended axially toward the free edge. This structural failure allowed the externally threaded locking ring and the power cylinder end seal block to shift outward along the ram. This displacement caused the end of the input feedback lever to break.
Metallurgical analysis determined that high cycle fatigue originated in the runout radius of the cylinder thread undercut. The crack had propagated over approximately 3,000 cycles, during which time four overload events occurred. The investigation also reviewed the history of similar failures in this type of PCU, noting that previous incidents had led to design improvements, such as modified thread radii and updated maintenance instructions.
Findings
- The primary cause of the failure was fatigue cracking within the thread undercut of the PCU casing.
- High-pressure loads, likely generated by the 'snubbing' action at the end of the actuator stroke, contributed to the fatigue.
- Although maintenance bulletins advised performing rudder checks slowly to avoid high snubbing loads, flight recorder data showed the crew performed one check in 3.5 seconds and another in 7.5 seconds, both of which were faster than the recommended 8-second minimum.
- The mechanical failure of the locking ring and seal block directly caused the hydraulic fluid loss and the physical breakage of the input feedback lever.